1. Field of the Invention
The present invention relates to FTTx systems (fiber to the x, x=Premises, Home, Business, etc.), and more particularly to a bidirectional optical triplexer applied to the FTTx systems.
2. Description of the Related Art
FTTH is an abbreviation of “fiber to the home”. Also, when ‘H’ of FTTH is replaced with ‘P’, FTTP means “fiber to the premises”. Although FTTx has different meanings depending on the last letter ‘x’ of FTTx, each of FTTH and FTTP has the same technical principle of transmitting data using optical fiber to a terminal connected to an optical communication network. FTTH system can be used to combine communication, broadcasting, and Internet data, and to integrate data, video, and voice information (called a “triple play”).
A bidirectional optical triplexer processes input/output signals of three wavelengths. An ONT (optical network terminal) included in a subscriber terminal processes a digital data signal having a wavelength of about 1490 nm and an analog video signal having a wavelength of about 1550 nm as input signals. A data signal having a wavelength of about 1310 nm is provided as an output signal. The output signal is transmitted to an OLT (optical line terminal) from the ONT.
FIG. 1 is a diagram showing a conventional bidirectional optical triplexer structure. An optical triplexer 100 includes a platform 120, first and second filters (FT) 130 and 140, first and second photodiode-transimpedance amplifier (PD-TIA) modules 150 and 160, a laser diode (LD) 170, and a monitoring PD (MPD) 180.
The platform 120 includes an optical path 125 optically connected to an external optical waveguide 110. The first and the second FTs 130 and 140 and the LD 170 are spaced from each other on the optical path 125.
The first filter (FT1) 130 is configured to reflect a first optical signal 190 that is an analog video signal having a wavelength of 1550 nm proceeding through the optical path 125. The first filter 130 transmits optical signals having wavelengths other than 1550 nm. The first filter 130 transmits a second optical signal 195 that is a digital data signal having a wavelength of 1490 nm and a third optical signal 175 that is a digital data signal having a wavelength of 1310 nm.
The second filter (FT2) 140 is configured to transmit the third optical signal 175 proceeding through the optical path 125 and to reflect an optical signal having a wavelength different from that of the third optical signal 175, i.e., the second optical signal 195.
The laser diode 170 outputs the third optical signal 175. The monitoring PD 180 monitors the third optical signal output from the laser diode 170.
The first optical signal 190 input to the internal optical path 125 is reflected by the first filter 130 and then is detected by the first PD-TIA module 150. The second optical signal 195 input into the second filter 140 after being transmitted by the first filter 130 is reflected by the second filter 140 and then detected by the second PD-TIA module 160. The third optical signal 175 output from the laser diode 170 sequentially transmitted through the second filter 140 and the first filter 130 is then output to the external optical waveguide 110.
However, the conventional optical triplexer 100 described above has a problem in that a size of the platform 120, in which components are integrated, is enlarged because of the various components that are required for the optical triplexer 100.
In this regard, if the laser diode 170 is physically positioned close to the first or the second PD-TIA module 150 or 160 in order to reduce the size of the platform 120, optical and electrical crosstalk occurs between elements. In this situation, the first or the second PD-TIA module 150 or 160 may falsely recognizes that an optical signal has been input.
In addition, since the filters cannot be perfectly manufactured, crosstalk may nevertheless occur. Ideally, the first optical signal 190 is completely reflected from the first filter 130. However, if a portion of the first optical signal 190 passes through the first filter 130, it is reflected by the second filter 140 and is then input to the second PD-TIA module 160. When this happens, the second PD-TIA module 160 falsely recognizes that the second optical signal is received, even though the second PD-TIA module 160 is not actually received, so a malfunction in the system occurs.
An analog video signal's intensity is generally about 10 dB larger than a digital data signal's intensity (even through it may vary depending on data format). This means that it is not possible to separate the optical signals by using only one filter. This, therefore, requires that another filter filtering operation mist be performed on the analog video signal.
Accordingly, there is a need in the art for an improved bi-directional optical triplexers.